The present invention relates to a device and method for evaluating low-μ road evaluation and a power distribution control device for four-wheel drive vehicles.
In a conventional method for evaluating low-μ roads, the wheel speed is detected, a slip rate for the wheels is calculated, a friction coefficient of the road surface based on the slip rate is estimated, and a low-μ road evaluation is performed based on the friction coefficient (conventional structure 1). In another method (conventional structure 2), the friction coefficient of the road surface is estimated solely on the basis of the driver's acceleration operation magnitude, and a low-μ evaluation is performed based on this road surface friction coefficient. “μ” here refers to the friction coefficient.
Japanese laid-open patent publication number Hei 11-148395 shows an example of a technology for performing low-μ road evaluations based on the slip rate and the accelerator setting (conventional structure 3). In this technology, each time the accelerator is operated, a slip count for when the slip rate exceeds a predetermined value is recorded, and a low-μ road evaluation is made when the counter for the acceleration operation exceeds a predetermined threshold value set up for each acceleration operation.
However, with a technology such as the conventional structure 1 that performs low-μ road evaluation based simply on the slip rate, evaluation errors can take place such as a low-μ road evaluation being made on a dry road surface when a sudden acceleration operation is performed to get the maximum throttle setting.
With a technology such as the conventional structure 2 that estimates the friction coefficient of a road surface based solely on the acceleration operations of the driver, evaluation errors can take place because of variations in the driver's acceleration operations.
Among four-wheel drive vehicles, stand-by four-wheel drive systems are well known. Stand-by systems switch between a four-wheel drive state and a two-wheel drive state as necessary and are equipped with a primary driving wheel connected directly to the internal combustion engine and a secondary driving wheel (also referred to as a driven wheel) connected to the internal combustion engine by way of a coupling. The power distribution to the secondary driving wheel is adjusted to provide an optimum setting by varying the coupling strength (engagement strength) according to the road surface state, the driving state, and the like.
In a four-wheel drive vehicle equipped with a low-μ road evaluation device as described above, four-wheel drive control is provided when there is a low-μ road evaluation and two-wheel drive control is provided when there is a high-μ road evaluation.
However, there are cases when stable control does not take place, such as when there is a road having low-μ surface scattered over it, e.g., a dry road with patches of snow. In this case, low-μ road evaluations and high-μ road evaluations would take place frequently, switching from one to the other at short intervals so that four-wheel drive control and two-wheel drive control would take place accordingly.
The conventional structure 3 seeks to overcome this problem by disabling low-μ road evaluation when slips in which the slip rate does not exceed a predetermined value take place consecutively a predetermined number of times (this is referred to as disabling method 1). Alternatively, the low-μ road evaluation is disabled when there is a continuous slip in which the slip rate does not exceed the predetermined value (this is referred to as disabling method 2).
However, the disabling method 1 and the disabling method 2 of the conventional structure 3 are not performed according to the magnitude of the friction coefficient. Thus, it is possible for low-μ road evaluation to be disabled regardless of the magnitude of the friction coefficient.